cicupoのC++日記

Thu 13, November (2008)

■[c++] Cheat Sheet (C/C++ const)

以下は、c でも c++ でも同じ。

X(x) になってるところは全て警告orエラーになる。

void foo(void*x){}

#define C const
#define X(x) /* x */

int main()
{
  int   *   *    p000;
  int   *   * C  p001;
  int   * C *    p010;
  int   * C * C  p011;
  int C *   *    p100;
  int C *   * C  p101;
  int C * C *    p110;
  int C * C * C  p111;

    foo(p000[0]);     foo(p000);     foo(&p000);
    foo(p001[0]);     foo(p001);   X(foo(&p001));
    foo(p010[0]);   X(foo(p010));    foo(&p010);
    foo(p011[0]);   X(foo(p011));  X(foo(&p011));
  X(foo(p100[0]));    foo(p100);     foo(&p100);
  X(foo(p101[0]));    foo(p101);   X(foo(&p101));
  X(foo(p110[0]));  X(foo(p110));    foo(&p110);
  X(foo(p111[0]));  X(foo(p111));  X(foo(&p111));

  return 0;
}

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idトラックバック

Mon 20, October (2008)

■[c++] valgrind

ある目的で, こんなテストプログラムを書いてみた.

#include <iostream>
#include <vector>

struct X
{
  std::vector<X*> children;
  X () { std::cout << "+[" << this << "]\n"; }
  ~X () {
    for( size_t i = 0; i < children.size(); ++i ){
      if( children[i] ){ delete children[i]; children[i] = 0; }
    }
    std::cout << "-[" << this << "]\n";
  }
};

int main () {
  X x;
  x.children.resize(2);
  x.children[0] = new X;
  x.children[1] = new X;
  x.children[0]->children.resize(2);
  x.children[0]->children[0] = new X;
  x.children[0]->children[1] = new X;
  return 0;
}

実行結果

$ ./a.out
+[0x7fff9877a320]
+[0x602030]
+[0x602050]
+[0x602090]
+[0x6020b0]
-[0x602090]
-[0x6020b0]
-[0x602030]
-[0x602050]
-[0x7fff9877a320]

とりあえず問題なく動くようだが, さて, 本当に大丈夫だろうか, と思い念のため valgrind を(人生で二度目に)使ってみた.

$ valgrind ./a.out
==25938== Memcheck, a memory error detector.
==25938== Copyright (C) 2002-2006, and GNU GPL'd, by Julian Seward et al.
==25938== Using LibVEX rev 1658, a library for dynamic binary translation.
==25938== Copyright (C) 2004-2006, and GNU GPL'd, by OpenWorks LLP.
==25938== Using valgrind-3.2.1-Debian, a dynamic binary instrumentation framewor
k.
==25938== Copyright (C) 2000-2006, and GNU GPL'd, by Julian Seward et al.
==25938== For more details, rerun with: -v
==25938==
==25938== Conditional jump or move depends on uninitialised value(s)
==25938==    at 0x40151F1: (within /lib/ld-2.7.so)
==25938==    by 0x4005052: (within /lib/ld-2.7.so)
==25938==    by 0x4007BA8: (within /lib/ld-2.7.so)
==25938==    by 0x400302E: (within /lib/ld-2.7.so)
==25938==    by 0x4013C14: (within /lib/ld-2.7.so)
==25938==    by 0x4001318: (within /lib/ld-2.7.so)
==25938==    by 0x4000A67: (within /lib/ld-2.7.so)
vex amd64->IR: unhandled instruction bytes: 0x66 0x66 0x66 0x66
==25938== valgrind: Unrecognised instruction at address 0x4015A71.
==25938== Your program just tried to execute an instruction that Valgrind
==25938== did not recognise.  There are two possible reasons for this.
==25938== 1. Your program has a bug and erroneously jumped to a non-code
==25938==    location.  If you are running Memcheck and you just saw a
==25938==    warning about a bad jump, it's probably your program's fault.
==25938== 2. The instruction is legitimate but Valgrind doesn't handle it,
==25938==    i.e. it's Valgrind's fault.  If you think this is the case or
==25938==    you are not sure, please let us know and we'll try to fix it.
==25938== Either way, Valgrind will now raise a SIGILL signal which will
==25938== probably kill your program.
==25938==
==25938== Process terminating with default action of signal 4 (SIGILL)
==25938==  Illegal opcode at address 0x4015A71
==25938==    at 0x4015A71: (within /lib/ld-2.7.so)
==25938==    by 0x4003C0E: (within /lib/ld-2.7.so)
==25938==    by 0x4013C14: (within /lib/ld-2.7.so)
==25938==    by 0x4001318: (within /lib/ld-2.7.so)
==25938==    by 0x4000A67: (within /lib/ld-2.7.so)
==25938==
==25938== ERROR SUMMARY: 1 errors from 1 contexts (suppressed: 0 from 0)
==25938== malloc/free: in use at exit: 0 bytes in 0 blocks.
==25938== malloc/free: 0 allocs, 0 frees, 0 bytes allocated.
==25938== For counts of detected errors, rerun with: -v
==25938== All heap blocks were freed -- no leaks are possible.
zsh: illegal hardware instruction  valgrind ./a.out

不正な命令って? なんだかおかしい.

でもこれは実は心配なくて, valgrind が新しめの命令に対応していないだけらしい. aptitude install valgrind/testing したら大丈夫になった.

《参考》

http://bbs.archlinux.org/viewtopic.php?pid=315064

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idトラックバック

Wed 15, October (2008)

■[c++] template？

下のような書き方で, 途中に出てくる template って何でしょうか?

template<T,S>
struct hoge {
  typedef typename foo<S>::bar Bar;
  typedef typename Bar::template xxx<T,S>::yyy zzz;
};

正解は, 以下のような文脈でテンプレートクラス(構造体)を参照するためのキーワード.

template<typename S>
struct foo
{
  struct bar
  {
    template<typename X,typename Y>
    struct xxx
    {
      struct yyy { };
    };
  };
};

template<typename T,typename S>
struct hoge
{
  typedef typename foo<S>::bar Bar;
  typedef typename Bar::template xxx<T,S>::yyy zzz;
};

int main(){
  hoge<int,int>  x;
}

template をつけずに

  typedef typename Bar::xxx<T,S>::yyy zzz;

と書いて良さそうに思うが, エラーが出る.

error: non-template ‘xxx’ used as template
note: use ‘typename foo<S>::bar::template xxx’ to indicate that it is a template

typename みたいなものか. コンパイラが struct hoge の宣言(あれ?定義?)を読んでいる段階では, まだ foo<S>::bar というクラスは実体化されていないので, xxx という名前がいったい何を指しているのか (型?変数?関数?) 不明であり, template かどうかも不明なので, template と書いてヒントを与えているわけか. なっとく.

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idトラックバック

Fri 10, October (2008)

■[c++] this はつけるべきか

C++ (や Java その他の言語)において, メンバ変数や関数のアクセスに this->をつけるべきか否か, という疑問があり, 調べてみた.

this をつけるべきかどうかについてはコーディング規約や好みの問題で議論されている(た). が, 何か決定的な違いがあったような気がしてもやもやしていた.

次の点は, 個人やプロジェクトの嗜好ではなく技術的にはっきりと意識すべき違いをもたらす.

#include <iostream>
struct X { void hoge () { std::cout << "hoge" << std::endl; } };

template <typename T>
struct A { void foo () { T x; x.hoge(); } };

template <typename T>
struct B : public A<T> { void bar () { foo(); } }; /* ※1 */

int main () {
  B<X> b;
  b.bar();
  return 0;
}

x.cpp: In member function ‘void B<T>::bar()’:
x.cpp:8: error: there are no arguments to ‘foo’ that depend on a \
template parameter, so a declaration of ‘foo’ must be available
x.cpp:8: error: (if you use ‘-fpermissive’, G++ will accept your \
code, but allowing the use of an undeclared name is deprecated)

B<X>::foo() は A<X>::foo() を継承しているはずなのに, コンパイラは foo の定義が見つからないといっている.

これを解決する手段は: